Method for reinforcing a thermoplastic resin composition

ABSTRACT

The present invention is directed to plant fiber-reinforced thermoplastic compositions and a method for reinforcing thermoplastic resins. The present invention provides a use for the cellulose portion of a plant material, which is the portion left over after processing the selected plant materials to separate the hemi-cellulose and lignin from the cellulose.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of application Ser. No. 13/648,738 filed Oct. 12, 2012.

FIELD OF THE DISCLOSURE

The present invention relates to a thermoplastic composite resin composition which includes plant fibers and a method for reinforcing thermoplastic resin compositions. More particularly, the cellulose component of plat material remaining after the removal of the hemi-cellulose and lignin components of the plant material is blended with one or more thermoplastic resins or to provide a reinforced resin composite.

BACKGROUND OF THE DISCLOSURE

The plastics industry is one of the largest consumers of organic and inorganic fillers. Inorganic fillers such as calcium carbonate, talc, mica and the like are well known, as well as organic fillers such as wood flour, chaff and the like, fibrous materials such as asbestos and glass fiber, as well as graphite, cokes, blown asphalt, activated carbon, magnesium hydroxide, aluminum hydroxide and the like. All of these additives have high specific gravities and their ability to improve physical properties of the composition is limited.

As an alternative to particulate fillers, thermoplastic materials can also be formed with fibrous materials to overcome those deficiencies. Fiber-reinforced composite materials based on thermoplastic materials are being increasingly used in many areas of technology in place of metallic materials as they promise a substantial reduction in weight, with mechanical characteristics which are otherwise comparable in many respects. For that purpose, besides the thermoplastic matrix, these composite materials include a fibrous component which has a considerable influence on mechanical characteristics, in particular tensile and flexural strength as well as impact toughness of the composite material. Fibrous components used are (i) fibers of inorganic materials such as glass, carbon and boron, (ii) metallic fibers, for example of steel, aluminium and tungsten, (iii) synthetic organic fibers, for example of aromatic polyamides, polyvinyl alcohols, polyesters, polyacrylates and polyvinyl chloride, or (iv) fibers of natural origin, for example hemp and flax.

The use of glass fiber-reinforced thermoplastic materials has of particular significance. In FIG. 1, a prior art process for the incorporation of glass fibers into a plastic resin, such as polypropylene, is illustrated. The polypropylene 10 is initially combined at a suitable temperature and pressure with the glass fibers 12 and other additives 14, as desired. The polypropylene 10, glass fibers 12 and additives 14 are mixed to form the composite material 16. This composite material 16 can be subsequently extruded at 18 for use in an injection molding process 20 to form a final molded product 22 having properties provided by the combination of the polypropylene 10 and glass fibers 12, along with any additional desired properties provided by the additives 14.

However, the production of glass fibers requires the use of considerable amounts of energy and the basic materials are not biological in origin so that the sustainability of the production process is open to criticism from ecological points of view. Furthermore, the disposal of glass fiber-reinforced thermoplastic materials is made difficult as even upon thermal decomposition of the material, considerable amounts of residues are left, which generally can only be taken to a disposal site. Finally glass fibers involve a high level of abrasiveness so that processing the materials in the context of usual processing methods for thermoplastic materials encounters difficulties.

Because of the above-mentioned disadvantages but also generally to improve the material properties therefore at the present time there is an intensive search for possible ways of replacing the glass fibers which dominate in many technical uses, as a reinforcing component. Organic fibrous materials of natural origin, such as plant materials appear to be particularly attractive in this connection because of their lower density and the reduction in weight that this entails in the composite material as well as sustainability and easier disposal.

The potential of using natural or plant fibers in plastic applications as a substitute for synthetic fibers such as glass, carbon, nylon, polyester, etc. has been recognized. For example, Kolla et al. U.S. Pat. No. 6,133,348, which is hereby expressly incorporated by reference herein, describes flax shives reinforced thermoplastic compositions and a method for reinforcing thermoplastic resins. The invention disclosed in Kolla provides a use for flax shives or particles in the thermoplastic compositions, which is the portion left over after processing plant materials to separate plant fibers (bast fibers) from the shives. The shives are the core tissue fibers which remain after the bast fibers are removed from the flax stem via the mechanical separation process disclosed in Leduc et al. U.S. Pat. No. 5,906,030, or other mechanical separation processes involving the hammering or bending of the natural plant fibers. These core tissue fibers include the cellulose, hemi-cellulose and lignin components of the flax fiber, along with a smaller portion of the woody bast fibers that remain on the shives, giving the shives a fiber purity of approximately eighty percent, at maximum.

It will be noted however that the use of natural fibrous materials as a fiber-reinforcing component can be confronted with worse mechanical characteristics in the resulting composite materials, in comparison with fiber-reinforced composite materials with glass fiber constituents. Furthermore natural fibers such as flax, hemp or also wood particles are of a fluctuating composition: individual batches of the material differ depending on the respective cultivation area, cultivation period, storage and possibly preliminary treatment. That means however that the mechanical characteristics of the fiber-reinforced thermoplastic materials to be produced also vary, which makes technical use thereof more difficult. The material can further change in form and appearance by virtue of progressing degradation processes. Finally, the constituent components of the various natural fibers can themselves create issues when the fibers are utilized in this manner. In particular, the hemi-cellulose fraction of natural fibers absorbs moisture, causing a detrimental effect on the dimensional stability and water resistance properties of any thermoplastic material to which the natural fibers are added.

As a result it is desirable to make use of the advantages linked to the use of organic materials of natural origin in creating composite materials, but by treating the natural fibers in a manner that improves the processing-relevant and use-relevant properties of the composite materials.

SUMMARY OF THE DISCLOSURE

According to one aspect of the present disclosure, fibers of natural or plant materials are used in the filling and reinforcement of formed composite materials including the fibers and thermoplastic matrix resins, such as polyethylene and polypropylene. The natural plant fiber materials to be used include cellulose, hemi-cellulose and lignin components or fractions. The fibers are treated prior to formation of the composite materials in order to separate the cellulose, hemi-cellulose and lignin fractions, such that the cellulose fraction or component of the natural fiber can be chemically treated and removed from the hemi-cellulose and lignin fractions. The fibers of the cellulose component of the plant materials can be substituted for the synthetic fibers used to at least achieve similar mechanical characteristics for the composite material as when synthetic fibers are used, in particular the tensile and flexural strength as well as impact toughness. In addition the use of the cellulose fraction of the natural plant materials does not absorb and retain water, and thus does not detrimentally affect the waterproof properties of the composite material. Further, the cellulose fraction of the natural plant component enables the composite material to be readily disposed of and/or recycled.

According to another aspect of the present disclosure, the natural plant fibers are mechanically treated prior to chemical treatment in order to obtain relatively pure plant material for use in the chemical extraction process. The particular mechanical treatment or decortication is accomplished in a manner that reduces the break age of the core fibers, resulting in longer cellulose fibers from the chemical extraction process, that in turn provide a stronger composite composition with enhanced strength and lighter weight than glass fiber filled composite materials.

Numerous additional objects, aspects and advantages of the present invention will be made apparent from the following detailed description taken together with the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the best mode of practicing the present disclosure.

In the drawings:

FIG. 1 is a schematic view of a prior art composite material production process; and

FIG. 2 is a schematic view of a composite material production process according to the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Referring now to the drawing figures in which like reference numerals designate like numerals throughout the disclosure, FIG. 2 illustrates a process for the formation of a product 116 created using a composite material 102.

The composite material 102 is formed of a thermoplastic resin or material 104, which is the term used to denote polymer materials which are soft or hard at the temperature of use and which have a flow transitional range above the temperature of use. Thermoplastic resins or materials comprise straight or branched polymers which in principle are capable of flow in the case of amorphous thermoplastic materials above the glass transition temperature (T_(g)) and in the case of (partly) crystalline thermoplastic materials above the melting temperature (T_(m)). They can be processed in the softened condition by pressing, extruding, injection moulding or other shaping processes to afford shaped and moulded parts. The thermoplastic material 104 used in the present disclosure can be any suitable thermoplastic resin material or combination of multiple thermoplastic materials, such as a plastic including one or more natural or petroleum based thermoplastic resins such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyacryl nitrite, polyamides, polyesters. polyacrylates and Poly Lactic Acid (PLA), among others. The thermoplastic material does not have to be a homopolymer but can also be in the form of a copolymer, a polypolymer, a block polymer or a polymer modified in some other fashion. Polypropylene is a particularly useful thermoplastic material for use in forming the composite material 102 of the present disclosure.

In addition to the thermoplastic material 104, the composite material 102 includes cellulose fibers 106. These fibers 106 can be obtained from any suitable natural plant material 109, such as natural fibrous plant materials including a) seed fiber plants, in particular linters, cotton, kapok and poplar down, b) bast fiber plants, in particular sclerenchyma fibers, bamboo fibers, (stinging) nettles, hemp, jute, linen or flax (fibre flax and oil seed flax), and ramie, c) hard fiber plants, in particular sisal, kenaf and manila, d) coir, and e) grasses. Bast fiber plants, such as flax and hemp, are particularly useful natural non woody, plant materials from which the cellulose fibers 106 can be obtained.

The bast plants include outer bast fibers that run longitudinally along the length of the plants and core tissue fibers disposed within the outer bast fibers. Because the core tissue fibers are the desired fibers, the outer bast fibers must be removed prior to use of the core fibers. In removing the outer bast fibers, care must be taken to avoid damaging or breaking the core tissue fibers in order to maximize the length of the core tissue fibers. Thus in a first step the straw is ratter under controlled environmental conditions (e.g., field ratted, chemically ratted and/or water rated) followed by mechanically treating the bast plant materials, in which the plant materials are decorticated by shearing the bast fibers from the core tissue fibers, as opposed to hammering or bending/flexing the plant material as in prior decortication processes. By shearing the bast fibers from the core tissue fibers, the core fibers can be kept intact more readily, thereby maintaining the overall strength and length of the core fibers. Using this process, core fibers of approximately 95-98% purity can be obtained. In addition, both ratted and non-ratted plant material can be used in the decortications process to obtain a clean core tissue fiber that can be used for production of the composite material.

In each case, the core fibers of the natural fibrous plant materials 109 include cellulose, hemi-cellulose and lignin components. To obtain the cellulose fibers 106 utilized to form the composite material 102 from the natural plant material, the hemi-cellulose fraction 108 and lignin fraction 110 are separated from the cellulose fibers or fraction 106, such that a purified crystalline cellulose fraction 106 can be added to the thermoplastic material 104 to form the composite material 102.

To separate the cellulose fibers/fraction 106 from the hemi-cellulose fraction 108 and lignin fraction 110 of the natural plant material 109, any suitable process 111 can be utilized, such as those employed on natural plant materials 109 for paper pulping, e.g., soda or kraft pulping, among others. More specific examples of processes for the separation of the hemi-cellulose fraction 108 and lignin fraction 110 from the cellulose fibers 106 of the plant material 109 include those that utilize an alkaline material 113, examples of which are disclosed in Hansen et al. U.S. Patent Application Publication No. 2009/0306253 and Costard U.S. Patent Application Publication No. 2010/0176354, among others, each of which are hereby expressly incorporated by reference herein in their entirety.

One suitable example is an alkaline separation process shown in Costard U.S. Patent Application Publication No. 2010/0176354 where a natural plant fiber material 109 is solubilized in an alkaline manner and which is characterized in that the natural fiber material 109 is treated with an alkaline material 113 without being subjected to mechanical stress a) at a temperature of between 5 and 30° C. and then b) at a temperature of between 80 and 150° C., and is then optionally washed and/or dried.

The alkaline materials 113 that can be used are, among other suitable alkaline materials, alkali metal hydroxide, in particular sodium hydroxide or potassium hydroxide, alkali metal carbonates, in particular sodium carbonate or potassium carbonate, or alkali metal phosphates, in particular trisodium phosphate or tripotassium phosphate.

The fiber degradation takes place at a pH of approximately between 8 to 14, preferably 10 to 14, more preferably 11 to 12 in the cold process (step a)) and preferably at a temperature of between 10 and 30° C., preferably between 10 and 25° C., in particular between 15 and 25° C., more preferably between 15 and 20° C.

The cold treatment according to step a) takes place over a period of 10 minutes to 3 hours, in particular 15 minutes to 2 hours and preferably 30 minutes to 1 hour. The hot treatment used according to step b) of the natural fiber material also takes place between a pH of 8 to 14, preferably 10 to 14, more preferably 11 to 12, and preferably at a temperature of between 80 and 140° C., preferably between and 140° C., in particular between 90 and 135° C., more preferably between 100 and 135° C.

The hot treatment according to step b) takes place preferably over a period of 20 minutes to 1.5 hours, in particular 30 minutes to 1 hour and preferably 45 minutes to 1 hour. The concentration of alkaline material in water in steps a) and/or b is, based on the active ingredient (typically a solid), preferably in the range from 5 to 15 g/l, in particular 7 to 13 g/l, preferably 8 to 12 g/l, particularly preferably at about 10 g/l.

The process performed according to steps a) and b) effectively dissolves the hemi-cellulose fraction 108 and lignin fraction 110 from the natural plant material 109, which can subsequently be removed with the alkaline solution, leaving the cellulose fraction 106 behind for subsequent washing and drying to a desired moisture level, e.g., about 2% by weight or below.

The alkaline treatment according to the disclosure can be supported by adding excipients. Dispersants, complexers, sequestering agents and/or surfactants are suitable here. Water glass and foam suppressors can likewise optionally be used depending on the end-application. Other customary excipients can also be used. The addition of a complexer, dispersant and/or surfactant to the baths can accelerate and intensify the wetting of the fibers. The materials customarily used for these respective purposes in fiber treatment are suitable here.

When separated, the cellulose fibers 106 are at least 95% w/w pure cellulose fibers, i.e., the fibers 106 contain not more than about 5 weight percent of material other than cellulose, i.e., lignin and hemi-cellulose. Further, the cellulose fibers 106 have a mean fiber length of less than about 2 mm.

Once liberated from the natural plant material 109, the cellulose fibers 106 can be utilized to form the composite material 102. These fibers 106 can be colored easily as the fibers 106 are very light, i.e., almost white in color and the composite made out of these is odorless. Chemical treatment of fiber 106 affects the cellulose structure, e.g., decreasing crystallinity and increasing the amorphous structure. For example, the chemical treatment opens the bonds in the cellulose fraction or fibers 106 for interaction with the polymer matrix 104 in forming the composites 102. The composite material 102 of the present disclosure may mixed together and processed by extrusion, compression molding, injection molding, or any other similar, suitable, or conventional processing techniques for synthetic or natural biocomposites.

FIG. 2 shows one embodiment of the processing of the composite material 102 of the present disclosure. The ingredients of the composite material 102, i.e., a thermoplastic material 104 and the cellulose fibers 106, may be blended or compounded with one another in a manner effective for completely blending the cellulose fibers 106 with the thermoplastic material 104, such as in a suitable mixer, e.g., a high or low intensity mixer. Depending upon the particular composition of the thermoplastic material 104 and the cellulose fibers 106, the temperature of the mixer in one embodiment should be from about 140° C. to about 220° C. for the proper combination of the components to form the composite material. One example of a mixer effective for blending the fibers 106 and thermoplastic material 104 is a high intensity thermokinetic mixer. In these types of mixers, frictional energy heats the contents until they become molten, a process that takes seconds or minutes depending on the speed of the impeller. In another aspect of the invention, heat from an external source can be supplied to melt the thermoplastic material 104 and effect blending of the cellulose fibers 106. An example of a low intensity mixer is a ribbon blender.

The formulation of the composite material 102 can be tailored by modifying the amounts or ratios of the thermoplastic material 104 and the cellulose fibers 106 used to form the composite material 102 depending on the particular application and/or function for the composite material 102. Additives (including, but not limited to, flow enhancers, anti-oxidants, plasticizers, UV-stabilizers, foaming agents, flame retardants, etc.) are used in formulation to enhance the functionality of the composite product. To accommodate the particular use and corresponding required properties of the composite material 102, the blending of the polymers/thermoplastic material 104 and the fibers 106 can also be varied in temperature and pressure. In addition, the blending parameters and component ratios for the composite material 102 can be altered depending upon the particular pant material from which the fibers 106 are obtained. Examples of the polymers used as the material 104 include, but are not limited to acrylonitrile butadiene styrene, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyacryl nitrite, polyamides, polyesters, polyacrylates, other engineering plastics and mixtures thereof.

In some particular embodiments of the composite material 102, the weight ratios/percentages of the thermoplastic material 104 and the cellulose fibers 106 used in the formation of the composite material 102 range from 1-60%. The fibre loading in biocomposite for the following process can be varied from process to process. Exemplary fiber loading percentages according to various molding processes in which the biocomposite material 102 is used are as follows:

Extrusion products: 1-30% (product examples: pies, profiles)

Injection molding: 1-45% (product examples: small components)

Compression molding: 1-60% (product examples: kitchen cabinets, bicycle components)

Rotational molding: 1-30% (product examples: water tanks, large storage boxes)

Vacuuming forming/Thermoforming: 1-20% (product examples: packaging materials, cups, plates, boxes, building insulation)

In one particular embodiment, the mixing/extruding of the thermoplastic material 104 and the cellulose fiber 106 to form the composite material 102 is performed with a dry blender, mixer, parallel screw extruder. The parallel screws in the device serve to blend the fibers 106 homogeneously with the polymer 104, while also reducing the damage and/or breakage of the cellulose fibers 106 in the mixture forming the composite material 102. In addition, the parallel screws help to reduce the residence time of the composite material formulation 102 by increasing the speed of mixing of the components of the composite material 102 in the device.

As a result of the use of purified cellulose fibers 106 obtained via the mechanical and chemical processing described previously, the fibers 106 develop a molecular bonding with the thermoplastic material 104 when blended to form the composite material 102 which provides superior performance of to composite materials having only mechanical binding between the polymer and the reinforcing fibers. Without wishing to be bound by any particular theory, it is believed that this molecular bonding occurs as a result of the thermoplastic material 104 flowing into and filling the inside the modified fibers 106 during the mixing/extrusion process. The increase in the melting temperature of the biocomposite 102 indicates a possible polymerization effect of the fiber that diffuses or dissolves into the polymer in the composite and correspondingly increases the thermal resistance of composite. Due to the porous surface of the treated fiber, molten polymer matrix enters in to the porous fiber and interlocks with each other and to form a strong binding within the biocomposite 102. Further investigation is required to determine the exact nature of bond. In addition, polymer matrixes encapsulate the fibre and enhance the biocomposite strength and reduce the porosity and the formation of air pockets within the biocomposite. This molecular bonding between the fibers 106 and the thermoplastic material 104 significantly improves the properties of the composite material 102, e.g., mechanical properties including tensile and flexural strength as well as impact toughness, and thermal properties. The properties of the biocomposite 102 vary as a result of the fibre loading and the type of polymer and/or additives used in the formation of the biocomposite 102. This, in turn, enhances the functionality of products 122 formed of the composite material 102 and enable the products 122 to be used in a wider range of industrial applications than prior fiber-reinforced materials. Also, in conjunction with the reduction in processing time in the parallel screw device, the molecular bonding between the fibers 106 and the polymer 104 limits any significant reduction of inbuilt additives present in polymer/thermoplastic material 104. As a result, it is only necessary to supplement any required additives, such as bonding additives, present in the polymer 104 during the formulation of the composite material 102, as opposed to adding the entire amount of the additives outside of those contained in the polymer 104.

Once mixed/compounded, the melted composite material 102 can be allowed to cool to room temperature and then further processed by conventional plastic processing technologies. Typically, the cooled blend is granulated into fine particles. The fine particles are then utilized for extrusion 112, injection 114 and/or compression molding to form finished parts or products 116.

In an alternative embodiment, the mixer can be operated without heat, such that the thermoplastic material 104 and cellulose fibers 106, after being mixed together, are transferred to a feed hopper, such as a gravity feed hopper or a hopper with a control feed mechanism. Alternatively, the thermoplastic material 104 and the cellulose fibers 106 can be individually fed to the extruder without being previously mixed together. The feed hopper transfers the composite to a heated extruder 112.

The extruder 112 blends the ingredients under sufficient heat and pressure. Several well-known extruders may be used in the present invention, e.g., a twin screw extruder. The extruder 112 forces or injects the composite material 102 into a mold 114. In an exemplary embodiment, the flow rate of the extruder 112 may be between about 150 and 600 pounds per hour. In other embodiments, the flow rate may be higher or lower depending on the type and size of the extruder 112. The injection mold 114 may be made up of one or more plates that allow the composite material 102 to bond and form a shaped-homogeneous product 116. A typical plate may be made from hardened steel material, stainless steel material or other types of metals. A cooling system (e.g., a liquid bath or spray, an air cooling system, or a cryogenic cooling system) may follow the injection mold 114.

In the mixer, a number of optional processing aids or additives 115 can be added to the thermoplastic material 104 and the cellulose fibers 106. These processing aids or modifiers act to improve the dispersion of fibers 106 in the thermoplastic polymer material 104 and also help further prevent the absorption of water into the fibers 106 and improve the various thermal, mechanical and electrical properties of the composite material 102, e.g., the strength of the resulting composite material 102. The addition levels of the modifiers or compatibilizers used depends on the target properties. For example, where higher tensile and flexural strengths are desired, higher levels of modifier or compatibilizer will be required. A compatibilizer is not required to achieve higher stiffness.

In one particular example of the present disclosure, the composite material 102 includes an amount of an wear additive 115 selected from aluminium or copper powder, or combinations thereof to increase the wear properties and enhance the longevity of the final product 122.

With regard to the molding processes 120 used to form the final product 122, the composite material 102 improves the product 122 formed by these processes 120 through the reduction of the formation of pin holes and the porosity of the material product 122. Without wishing to be bound by any particular theory, it is believed that these results are achieved in the composite material 102 as a result of the close packing and increased density of the fibers 106, polymer 104 and additives 115 due to the properties of the cellulose fibers 106, and the consequent removal of entrapped air bubbles during the processing of the fibers 106 and thermoplastic material 104, along with the additives 115, to form the composite material 102. As a result, the final product 122 is more solid and stronger than products formed from prior fiber-reinforced materials.

Further, with the use of the cellulose fibers 106 formed in the above-described manner, it is possible to achieve higher grade properties (mechanical, thermal, electrical, etc.) for the final product 122 while using lower grade thermoplastic materials 104 in combination with the cellulose fibers 106. In particular, as a result of the properties and purity of the cellulose fibers 106, the fibers 106 can bond well with a wide range of grade of polymeric/thermoplastic materials 104 to achieve products 122 with the desired properties. Further, to address any issues presented by the particular polymer/thermoplastic material 104, the weight percentage or weight ratio of the fibers 106 Fine can be increased in formulation of composite material 104 without compromising the quality and desired properties of the final product 122. In addition, by increasing the amount of the cellulose fibers 106 utilized in the composite material 102, the consequent consumption of the polymer 104 will be reduced.

For a better understanding of the objects and advantages of the present invention, the same will be now described by means of several examples. However, it should be understood that the invention is not limited to such specific examples, but other alterations may be contemplated within the scope and without departing from the spirit of the invention as set forth in the appended claims.

While the formulation of the particular biocomposite material 102 depends on the final product 122 formed from the biocomposite material 102, its functionality, and/or as described above the particular molding process used to form the biocomposite material 102 into the final product 122.

In one example of biocomposite composition 102, the formulation includes:

a) natural/petroleum based thermoplastic material(s): 99-40% w/w

b) fiber 1-60% w/w

c) additives 1-5% w/w.

Biocomposite materials 102 of different grade (e.g., extrusion grade, injection grade, compression grade, rotational grade, vacuum forming grade) are manufactured by changing the formulation of the biocomposite material 102, and in one example by changing the amount of fiber 106 present and consequently adjusting the percentages of the remaining components.

One particular example of a thermoforming/vacuum forming formulation for the biocomposite material 102 is as follows:

a) polystyrene

b) treated natural fiber

c) butane

d) additives (zinc stearate, magnesium stearate)

e) talcum powder.

Other examples of biocomposite material 102 formed according to the present disclosure are found in the following tables.

TABLE 1 Properties Liner low density polyethylene - dicumyl peroxide pre-treated flax fibre Flax straw/Industrial Hemp stalk Chemically Composite Unretted Field retted Water retted retted properties Unit Flax Hemp Flax Hemp Flax Hemp Flax Hemp Melt Flow g/10 min 2.8 2.6 3.7 3.5 4.1 3.4 3.8 3.5 Index Melting point ° C. 130 128 129 127.4 130.1 128 130.6 129 Tensile Mpa 13.2 15.3 17.6 16.9 18.3 18.7 22.2 21 Strength Tensile Impact KJ/m² 178 172 188 182 194 178 223 205 strength Hardness SD 12 11 17 18 18 17 23 21 Water % 3-5 2-6 <1 <1 <1 <1 <1 <1 absorption@50 RH

TABLE 2 Properties Liner low polyethylene - triethoxyvinylsilane pre-treated flax fibre Flax straw/Hemp stalk Chemically Composite Unretted Field retted Water retted retted properties Unit Flax Hemp Flax Hemp Flax Hemp Flax Hemp Melt Flow g/10 min 2.0 2.2 2.7 2.4 2.6 2.4 2.8 2.4 Index Melting point ° C. 129 131.2 128.6 129 129 129 129 129.6 Tensile Mpa 15 14.2 18.4 17.1 20.1 17.4 19.3 17.9 Strength at Yield Tensile Impact KJ/m² 178 161 188 186 199 193 218 209 strength Hardness SD 9 11 14 15 19 19 20 18 Water % 3-5 2-6 <1 <1 <1 <1 <1 <1 absorption@50 RH

TABLE 3 Properties High density polyethylene - benzoyl chloride pre-treated flax fibre Flax straw/Hemp stalk Chemically Composite Unretted Field retted Water retted retted properties Unit Flax Hemp Flax Hemp Flax Hemp Flax Hemp Melt Flow g/10 min 1 1.2 1.6 1.5 1.8 1.7 1.8 1.4 Index Melting point ° C. 130 128 130 130 129 130 129 130 Tensile Mpa 16.3 13.7 16.3 16.2 18 18.1 23.4 19.2 Strength at Yield Tensile Impact KJ/m² 167 157 177 179 188 185 221 178 strength Hardness SD 17 11 12 15 19 22 21 19 Water % 3 2 <1 <1 <1 <1 <1 <1 absorption@50 RH

TABLE 4 Properties High density polyethylene - dicumyl peroxide pre-treated flax fibre Flax straw/Hemp stalk Chemically Composite Unretted Field retted Water retted retted properties Unit Flax Hemp Flax Hemp Flax Hemp Flax Hemp Melt Flow g/10 min 0.5 0.8 1.0 1.5 1.2 1.6 1.6 1.5 Index Melting point ° C. 130 126 131.6 128.4 128 129 129 128 Tensile Mpa 15 14.3 16.8 15.4 17.5 18.1 24.1 21.2 Strength at Yield Tensile Impact KJ/m² 180 167 197 180 185 185 220 180 strength Hardness SD 13 9 14 12 15 12 17 15 Water % 3 2 <1 <1 <1 <1 <1 <1 absorption@50 RH Oilseed flax and industrial hemp fiber has promising future in the plastic industries. It is observed that unretted and chemically retied flax and hemp can be used in plastic composite (LLDPE and HDPE). Chemically retted fiber increased the T_(m) of composite compared to pure polyethylene. The increase of T_(m) may be attributed to the polymerization effect of the fiber that diffuses or dissolves into the polymer in composite and increased the thermal resistance of composite. This investigation indicated that chemical retting has a great influence on mechanical properties of (flax and hemp) polymer composites products developed through rotational molding processes.

Various other alternatives are contemplated is being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention. 

We claim:
 1. A method for reinforcing a thermoplastic resin composition comprising: a) providing an amount of cellulose fibers obtained by the separation of the cellulose fiber fraction from hemi-cellulose and lignin fractions of a natural plant material; and b) blending from about 1 to about 60 weight percent of the cellulose fibers based on the weight of the composition with a thermoplastic resin, the cellulose fibers having a porous surface obtained by the mechanical shearing and subsequent chemical separation of the cellulose fiber fraction from hemi-cellulose and lignin fractions of a natural plant material.
 2. The method of claim 1 wherein the natural plant material is selected from the group consisting of natural fibrous plant materials including a) seed fiber plants, linters, cotton, kapok and poplar down, b) bast fiber plants, sclerenchyma fiber plants, bamboo fiber plants, (stinging) nettles, hemp, jute, linen or flax, and ramie, c) hard fiber plants, sisal, kenaf and manila, d) coir, and e) grasses.
 3. The method of claim 1 wherein the cellulose fibers are formed of at least 95% pure cellulose fibers.
 4. The method of claim 1 wherein the thermoplastic resin is selected from the group consisting of polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyacryl nitrite, polyamides, polyesters, polyacrylates and mixtures thereof.
 5. The method of claim 1 wherein the step of providing the cellulose fibers comprises: a) mechanically separating core tissue fibers from outer plant fibers; and b) chemically separating the cellulose fibers from hemi-cellulose and lignin fractions of the core tissue fibers.
 6. The method of claim 5 wherein the step of mechanically separating the outer plant fibers from the core tissue fibers comprises shearing the outer plant fibers from the core tissue fibers to minimize damage to the core tissue fibers.
 7. The method of claim 1 wherein the step of blending the cellulose fibers with the thermoplastic resin comprises mixing the cellulose fibers and the thermoplastic resin in parallel screw mixing device to minimize breakage of the cellulose fibers and the residence time of the fibers and resin in the mixing device.
 8. The method of claim 1 wherein the step of blending the cellulose fibers with the thermoplastic resin comprises forming molecular bonds between the cellulose fibers and the thermoplastic resin. 